Feeling pretty confident from that it was the TV power supply? Would the HOT or bridge diode going cause it to short out to the video ground or otherwise explain it flowing all the way back up the inputs to the PC?
CRT TV techs are in short supply these days... I know about enough to get myself in trouble but am a quick study. Haven't had time to take the old board out yet, planning to check the HOT and bridge diode first, and check its power cable for shorts. Not sure how to check the posistor, and really unclear how to effectively check the flyback. Though, I can't seem to quite figure out what is the HOT or equivalent on this board... the service manual suggests going from the fuse, to the bridge, and then to IC801S, which is a W6750F... which may be what is serving the function of the HOT, from my limited understanding...? Though with its more complicated pin-out, I don't know how to test that.
Service manual: https://elektrotanya.com/samsung_txt2782qx-xaa_chassis_k64c_sm.rar/download.html#dl
CRT TV techs certainly are hard to find these days. I reasoned this would be the case about a dozen years ago, and have started teaching myself TV & electronic repair, slowly at first. I have learned a lot about CRT TVs and arcade monitors, maybe more than I expected, and other things too. That out of the way, I said "your TV needs a tech" because I don't know everything and its easier if you can test and intuit some things directly. I'll try to help but that is my disclaimer.
Your initial analysis and approach to finding the issues seems about right. There is a pic in the troubleshooting guide with parts marked, but some of the labels are confusing and the boxes marking where things are is wonky. So I've attached my own pic below.
The "diode bridge" D801S is the bridge rectifier chip, which is in early stage of SMPS. It is really 4 diodes bundles together into a single chip. Looks like a long rectangle with a corner cut off and has four legs. I've marked the DIODE BRIDGE/BRIDGE RECTIFIER in yellow
YELLOW.
IC801S is your SMPS main transistor - this is where the "switching" in "switch-mode power supply SMPS" comes from. It is some kind of MOSFET, fast-switching power transistor.
IC801S is in red, on that big heat sink. You can test it by connecting an oscilloscope to pin 1 and comparing the waveform to that marked in the troubleshooting guide (TP01).
The horizontal output transistor (HOT) is always close to the flyback and the horizontal deflection. I've marked the
HOT in blue. A failing HOT *could* cause the ground leakage issues you saw, I guess. But other than that I would be mostly looking at the SMPS. Testing the HOT is not hard but tricky, can get into that later.
Another thing that I'd look at, maybe first because they are easy to test, is the small bunch of
main power diodes I've marked in
turquoise. The main power diodes failing is a common problem. It is also a good place to look for testing DC secondary voltages including the B+, as this is where they all come out from the main transformer. Even the flyback and the HOT rely on these voltages being right.
EDIT: You can measure the B+ at cathode end of D811 above (more convenient), or flyback pin 3.
So that is why I think your problem is most likely in the SMPS, and why I'd look there first. I'd be tempted to check and/or replace one or both of the D801S and IC801S. I'd also go and check all the smaller diodes around them because it is pretty easy.
Ideally you'd use an isolation transformer (for safety) and a variac (variable voltage transformer). Then you could replace fuse and use the light bulb in series trick while slowly raising the power. With power applied you could test voltages and hopefully see what parts are failing. Or you could try just replacing the fuse and test a few voltages to see where the problem is, but a little risky and you never know when it might blow again (or worse).
If you want to get into this repair stuff it helps if you have a bunch of "jellybean" parts like the bridge rectifiers, diodes, capacitors, common ICs, HOTs etc & so on to swap in/out. A lot of pro repairers fix thing by just identifying problem areas and replacing suspect parts, rather than testing everything. ICs for example can be hard to test properly unless they exhibit dead shorts. Mostly you just guess it has failed because other bits relying on it aren't working.
I'd also do a quick check of the electro caps health with an inspection and an ESR meter (Dick Smith). For checking flybacks I use a LOPT/flyback/rings tester (Dick Smith). I got a few of these two meters, in kit form, from Dick Smith before the company folded in mid-2000's. Still have some here, "new-in-box" and plastic wrap. Let me know if you are interested.